1. Field of the Invention
The present invention relates to an image forming apparatus, and, more particularly, to an image forming apparatus for image formation by scanning a recording medium with a light beam such as a laser beam modulated with pixel signals.
2. Description of the Prior Art
As disclosed in the U.S. Pat. No. 4,233,612, there are already proposed various image forming apparatus for the formation of images, such as characters or graphics, by scanning a recording medium, such as an electrophotographic photosensitive member, with a light beam, such as a laser beam, modulated in response to information signals.
In such an image forming apparatus, there is proposed a multi-beam scanning method utilizing plural light beams in order to achieve a high-speed scanning.
In case of a high-powered light beam source, such as a gas laser, it is possible to form plural beams from a single beam by means, for example, of an acousto-optical modulating element and to modulate in order of their formation the formed plural beams for scanning. Such a method, however, is not suitable for a compact image forming apparatus since it not only requires the large-sized light beam source and the additional devices such as the acousto-optical modulating element, but involves a large complicated scanning mechanism.
On the other hand, there are known small light beam sources, such as semiconductor lasers, which can control the light beam in response to the information signals without additional devices, such as the acousto-optical converting element, and can, therefore, be utilized in the compactization of the apparatus.
However in a semiconductor laser device capable of generating plural beams, the light emitting points are at least mutually distanced, approximately by 100 microns, so that the scanning beams are inevitably spaced by 1 to 3 millimeters on the recording medium if an ordinary optical scanning system is employed. Since ordinary image recording requires at least 6 or preferably 8 scanning lines of light beam per millimeter, the multiple beam scanning with such a beam source will require complete control. More specifically, in a scanning cycle with simultaneously running plural beams, while a first light beam is modulated according to the information of first line of an original document, a second light beam has to be modulated according to the information of a line which is several lines ahead of the second line of the original document. In the subsequent scanning cycle, while said first beam is modulated according to the information of the second line, said second beam has to be modulated according to the information of a line further ahead.
Consequently, the control unit for the multiple beam scanning requires sufficient memory means for storing a large amount of information to be recorded and means for controlling complicated data readout from said memory means according to the line to be scanned.
In order to avoid the above-mentioned difficulty, the U.S. Pat. No. 4,253,102 discloses a method in which the light-sources are not perpendicularly arranged, but are obliquely arranged with respect to the principal scanning direction. In such a method, however, a complicated control for synchronization is inevitable as the light beams have different start timings.